Applied Surface Science 255 (2009) 5264–5266
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Fabrication of Mg-doped ZnO thin films by laser ablation of Zn:Mg target Tae Hyun Kim, Jin Jae Park, Sang Hwan Nam, Hye Sun Park, Nu Ri Cheong, Jae Kyu Song *, Seung Min Park * Department of Chemistry, Kyunghee University, Seoul 130-701, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Mg-doped ZnO thin films were fabricated by laser ablation of Zn:Mg targets consisting of Mg metallic strips and Zn disk in oxygen atmosphere with a goal to facilitate convenient control of Mg contents in the films. The characteristics of the deposited films were examined by analyzing their photoluminescence (PL), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) spectra. Mg contents as analyzed by XPS indicate that the target composition is fairly transferred to the deposited films. The wurtzite structure of ZnO was conserved even for the highly doped ZnO films and there was no Mg- or MgO-related XRD peaks. With increase in the Mg content, the bandgap and PL peak energy shifted to blue and the Stokes shift became larger. ß 2008 Elsevier B.V. All rights reserved.
Available online 26 July 2008 PACS: 79.20.Ds 52.50.Jm Keywords: Pulsed laser deposition ZnO Mg-doping
1. Introduction Wide bandgap materials such as GaN, ZnSe, ZnS, SiC, and ZnO have drawn great attention due to their applications in shortwavelength photonic devices. Among these materials, GaN preceded the others in being commercialized as blue lightemitting diode and laser diode. Recently, however, ZnO has been extensively studied because of its unique advantages: (1) large exciton binding energy (60 meV) at room temperature, (2) high radiation hardness, and (3) ability to make alloy with MgO [1]. In particular, the bandgap of ZnO can be easily controlled over a wide range by making MgxZn(1 x)O alloy thin films, which suggested the feasibility of ZnO as a strong candidate to replace GaN for blue light-emitting materials. ZnO films have been grown by various techniques including chemical vapor deposition, vapor phase epitaxy, molecular beam epitaxy, RF sputtering, and pulsed laser deposition (PLD) [2]. PLD is one of the most convenient techniques to achieve easy control of dopant level in ZnO films; the composition of a target material is readily transferred to the film and the gas phase dopant can also be incorporated into the films via reactive laser ablation process. Doping of ZnO films has been mostly performed by laser ablation of ceramic targets prepared by mixing metal oxide powders [3]. Mgdoped ZnO films on glass substrate have been grown by PLD using a ceramic target previously [4].
* Corresponding authors. Tel.: +82 2 961 0226; fax: +82 2 957 4856. E-mail addresses:
[email protected] (J.K. Song),
[email protected] (S.M. Park). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.105
However, ceramic targets so prepared have unavoidable disadvantages: (1) targets with various mixing ratios need to be prepared according to the prescriptions in order to adjust the dopant concentrations in the films, (2) the ceramic targets made by cold-press of metal oxide mixtures followed by sintering are less preferred than metal targets because the ceramic targets produce more particulates than metal. In order to facilitate the control of dopant levels and minimize particulate problems, we have designed Zn:Mg targets for PLD, where Mg strips are attached on the surface of metallic Zn disk. This certainly enables much easier control of Mg concentrations in the deposited films compared to the ceramic targets. Previously, we had successfully grown silicon-rich silicon oxide films doped with Er using Si:Er target [5]. Here, undoped and Mg-doped ZnO thin films with Mg concentration ranging from 5 to 30 mol% were fabricated by PLD using Zn:Mg targets, which were analyzed by XRD, UV–vis, PL, and XPS. All the deposited films showed preferred orientation of ZnO (0 0 2), and the blue shift of the PL peak with increase in Mg concentration was apparent. 2. Experimental Zn:Mg targets were prepared by attaching Mg strips by carbon tapes on Zn disks (Niraco, 99.999%) with size of 25.4 mm in diameter. The concentrations of Mg in the deposited films were varied by changing either size of Mg strips or their numbers. A Qswitched Nd:YAG laser (l = 355 nm, Quantel, YG980C) operating at 10 Hz was loosely focused onto the target placed in the PLD chamber using a lens (focal length 30 cm) with an angle of incidence of 458.
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The laser fluence at the target surface is estimated to be 2.4 J/cm2. A fused silica substrate (10 mm 10 mm) was mounted facing the target which was rotated to minimize target aging effects. The distance between the target and the substrate was typically 23 mm. PLD was performed for 30 min at 600 8C. Oxygen gas (99.999%) was fed to the PLD chamber using a needle valve and the typical oxygen pressure was 133 Pa as measured by a full range gauge (Balzers PKR250). The deposited films were characterized by XRD, UV–vis, and XPS. Also, the photoluminescence spectra were obtained using a He–Cd laser (325 nm, Kimmon IK 3252R-E) as an excitation source at room temperature. 3. Results and discussion The contents of Mg in films as analyzed by XPS were roughly proportional to those of the targets. The concentrations of Mg in the films and the targets do not coincide exactly due to the different laser ablation efficiencies of Zn and Mg in the Zn:Mg target as well as the oxygen incorporation into the films via reactive laser ablation. Also, the relative enrichment of Mg over Zn in the films compared to the target is attributed to the higher vapor pressures of Zn and ZnO than those of Mg and MgO at the deposition temperature [6]. Since the content of MgO is limited to 4 mol% for the ZnO–MgO binary system, the PLD-grown Mg-doped ZnO films are considered to be metastable phase. Note that we adopt the contents of Mg in the targets, not in the films, in the following figures. Fig. 1 shows PL spectra for the Mg-doped ZnO films deposited at 133 Pa oxygen pressure for different Mg contents ranging from 0 to 30%. The peak of near band edge (NBE) emission shifted to blue with increase in the content of Mg, while the intensity of NBE emission decreased dramatically. On the other hand, the red emission which has to do with the defects in the films increased substantially with the Mg content. As described by Shan et al. [3], the blue shift of NBE emission originates from Burstein–Moss effect: (1) the doping of ZnO films with Mg increases the carrier concentration; (2) the increase in the doner concentration results in the occupancy of the states in the conduction band, namely the upward movement of the Fermi level; (3) the filling of the conduction band brings about the blue shift of the NBE emission as the low-energy transition is circumvented. The bandgap and PL peak energies of Mg-doped ZnO films at various Mg contents are shown in Fig. 2. The inset illustrates
Fig. 1. PL spectra for the undoped and Mg-doped ZnO films. Mg contents in the targets ranged from 0 to 30%, which were varied by changing the number of the Mg strips or their area.
Fig. 2. The bandgap and PL peak energy for the films deposited by laser ablation of Zn:Mg targets with Mg contents ranging from 0 to 30%. The difference between the two energies corresponds to the Stokes shift. The inset shows the plot of (ahn)2 vs. photon energy, where a is the absorption coefficient. The Mg contents for (a), (b), (c), (d), (e), and (f) are 0, 5, 10, 15, 20, and 30%, respectively.
ultraviolet absorption spectra presented as plots of (ahn)2 as a function of photon energy, where a is the absorption coefficient. The absorption onset is clearly shifted to blue with increasing Mg content, which indicates an increase in the bandgap energy. From the slopes of each plot, we deduced bandgap energies of the films [7]; as Mg content increased from 0 to 30%, the bandgap increased nearly linearly from 3.26 to 3.73 eV. The blue shift of NBE emission from the PL peak energy was also apparent but the relative rate of its increase with Mg content is lower than that of the bandgap energy. Namely, the Stokes shift, defined as the difference of PL peak and bandgap energy [8], increased with Mg content and reached 158 meV, while it was as small as 18 meV for the undoped ZnO film. Only ZnO-related peaks such as (0 0 2) and (0 0 4) were observed and MgO- or Mg-related peaks were not detected. This leads us to conclude that the excess concentration of Mg and MgO, if any, exists in the form of amorphous phase. It is clear that both films have preferred orientation of ZnO (0 0 2), indicating a perpendicular alignment of the c-axis of the grains, and the wurtzite structure
Fig. 3. XRD spectra showing ZnO (0 0 2) peaks for the films deposited by laser ablation of Zn:Mg targets with Mg contents ranging from 0 to 30%. The inset shows the shifts of the (0 0 2) peaks with the increase in Mg content.
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at 530.0, 530.9, and 531.8 eV, each representing ZnO, MgO and Zn(OH)x, respectively. Water molecules adsorbed on the deposited films have been reported to dissociate into H and OH to form Zn(OH)x, which is initiated by the defect sites on the film surface [2,10]. The relative contribution of Zn(OH)x is higher for the films containing more Mg which are expected to have more defect sites on the surface. Formation of Zn(OH)x or ZnOx(OH)y, however, is possible in the inner layers and may not be limited to the surface sites [11]. 4. Conclusions
Fig. 4. O 1s XPS spectra for the films deposited by laser ablation of Zn:Mg targets with Mg contents ranging from 0 to 30%. The three resolved bonding components at 530.0, 530.9, and 531.8 eV correspond to ZnO, MgO, and Zn(OH)x, respectively.
of ZnO is conserved after high-level doping [4]. Fig. 3 shows the XRD spectra of ZnO (0 0 2) peaks for the Mg-doped ZnO films with different Mg concentrations. For the undoped film, the ZnO (0 0 2) peak was observed at 34.58. This angle is slightly larger than that of the films grown at lower temperatures, which reflects the residual stress stemming from the different thermal expansion coefficients between the film and substrate [9]. The (0 0 2) peak shifted towards higher diffraction angles as Mg concentration increased. This conforms to the expectation from Bragg’s law; as Mg2+ ions with radius, 0.65 A˚ replace Zn2+ ions with larger radius, 0.74 A˚, the contraction of the unit cell occurs. The line width of the (0 0 2) peak, 0.228 for the undoped ZnO film, increased roughly with the doping level and reached 0.268 for 30% Mg concentration, which implies that the grain size in the ZnO film is reduced as it is doped with Mg. The average grain size estimated by Scherrer’s formula is around 130 nm, with the Scherrer constant assumed to be 1, for the undoped ZnO film [2]. The broadening of the (0 0 2) peak at higher levels of Mg can also reflect the expansion of the unit cell due to the occupancy of interstitial sites by Mg2+ ions [7]. The results of XPS analysis carried out for the undoped and Mgdoped ZnO films are shown in Fig. 4. In XPS study, the C 1s peak (284.6 eV) was taken as a reference for the calibration of the binding energies. The O 1s core-level spectra have pronounced shoulders at higher binding energies, consisting of three sub-peaks
We have devised simple Zn:Mg targets consisting of Zn disk and Mg strips and demonstrated that Zn:Mg targets can be successfully employed to deposit Mg-doped ZnO thin films with various concentrations by pulsed laser deposition with a goal to facilitate bandgap engineering. The Mg contents in the deposited films were varied simply by changing the area covered by the Mg strips on the Zn disk surface. All the films so prepared were oriented to the caxis of wurtzite ZnO structure and no other crystalline features related to Mg and MgO were observed. The average grain size for the undoped ZnO film was about 130 nm and it did not change noticeably by high-level doping of Mg. The blue shift of the bandgap and PL peak energy with Mg content in the film was quite distinct and the Stokes shift as large as 158 meV was observed for the Mg-doped ZnO film grown by a target with 30% of Mg. The O 1s peak in the XPS spectra consists of three peaks corresponding to ZnO, MgO and Zn(OH)x, where the formation of Zn(OH)x is attributed to dissociation of H2O at the defect sites. Acknowledgment This work was supported by Grant No. R01-2006-000-10396-0 from Korea Science and Engineering Foundation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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